1. Field of the Invention
The present invention relates to measuring beam deflection errors, and in particular to a method and structure for electronically measuring scan deflection distortion.
2. Description of the Related Art
The manufacture of electronic integrated circuits relies heavily on the use of image projection techniques to expose resist-coated wafers with light or X-rays. The patterns formed by this exposure determine the various circuit connections and configurations. In certain applications, integrated circuit patterns are written directly onto resist-coated wafers in a process called electron-beam direct write.
In any exposure method, accuracy of the projected image is a prime consideration. This accuracy is particularly important in the manufacture of high density Random Access Memories (RAM) in which yield and ultimately the cost of the components depend heavily on meeting tight exposure placement requirements. Presently, electron beam lithography systems provide the most accurate method of placing the exposure patterns onto substrates for image projection techniques and for direct write techniques. A substrate is defined as a mask, wafer, or any similar material used in a semiconductor process. In electron beam lithography systems, beam position during exposure is critical to achieving and maintaining the tight performance tolerances required.
The electron beam position is conventionally controlled via a technique called raster scanning. In this method, the electron beam is repeatedly deflected in a continuous series of ramp deflections and flyback periods similar to a scanning technique used in televisions. Typically, the electron beam is deflected as rapidly as possible to minimize the time required to completely expose a pattern. In this manner, the production rate (i.e. throughput) is increased, and the unit cost per mask or wafer is lowered.
In a conventional system, the electron beam, which is typically a spot from 0.1 micron to 1 micron in diameter, is deflected over a range of several hundred microns and must be positioned within that range with an error of less than 0.01 microns, i.e. preferably a placement error of less than 1 part in ten thousand. This low placement error, coupled with a typical scanning rate of 30 kilohertz (KHz), make direct measurements of such signals exceptionally difficult.
Conventionally, the deflection signal is measured by its product, i.e., the patterns on the mask or wafer. Non-ideal pattern placement seen on the mask or wafer is identified through a series of measurements and tests, and appropriate modifications are then made to the deflection signal to correct these deficiencies. This method of modifying the performance of the deflection signal relies heavily on standard procedures of writing a pattern, processing the mask or wafer, and then measuring the accuracy of the patterns with proven, but relatively slow, metrology techniques. This process may require from one hour for a simple calibration to several weeks for a full calibration.
Preferably, calibration is performed without invoking the time-consuming process of writing and reading actual exposures of masks and wafers. "Real-time" characterization of electron beam parameters is achieved by scanning the electron beam at very low frequencies over a reference grid with known positions. This technique is well-known in the art and provides sufficient information to functionally calibrate an electron beam lithography system.
However, the correlation between these low frequency measurements and the actual beam writing deflection signal is not exact. Specifically, the high frequency writing signal introduces several anomalies in the electron beam which require further characterization via extensive pattern writing and reading. These anomalies include deflection axis crosstalk, slight deflection axis rotation effects, gain differences, and scan offsets, all of which vary as a function of scan frequency.
Because electron beam parameters are difficult to precisely predict via the above-described low frequency scanning technique, the previously-noted time-consuming method of writing and reading actual exposures is presently the only method to fully characterize the writing performance of lithography systems.